In the snowplow systems shown in the above patents, the blade is supported on a vehicle using a pivotal mounting frame allowing an operator to pivot the blade about a vertical axis and thereby to selectively direct the plowed snow to either side of the vehicle path. Such mounting frames are generally mounted on a vehicle for pivoting about a horizontal axis near the front of the vehicle whereby the plow blade may also be selectively raised and lowered using hydraulic controls mounted on the vehicle. The plow blade itself is typically attached to the mounting frame for limited rotation about a horizontal mounting axis near the plow blade and is further equipped with springs connected between the blade and the mounting frame to dampen this rotational movement and to bias the blade to an upright position. In addition, the blade may optionally be offset from the ground using skids or casters mounted to the mounting frame. This is particularly desirable for heavy highway plows to minimize damage to the road surface. The above patents illustrate various structures by which snowplow blades have heretofore been supported on such mounting frames. In Miceli U.S. Pat. No. 3,828,449, the blade is mounted on a U-shaped support member using trunnions 40 and 42 as shown in FIG. 2. The support member provides manual adjustment of the blade angle about a center pin 46 and lock 48. Springs 44 are connected between the blade and the support member to hold the blade upright and to dampen the pivoting of the blade about the trunnion axis as the blade contacts obstacles. Miceli U.S. Pat. No. 3,706,144 also illustrates an arcuate snowplow blade support member in FIG. 1. In this arrangement, the arcuate member forms a sector and a long pin pivotally interconnects the blade and support member. Hydraulic control of the blade angle is provided using rams 2 and 3. In King U.S. Pat. No. 3,353,287, the snowplow blade support system includes a C-shaped channel member pivotally attached to the blade with pins, forming a sector arrangement with an arcuate plate member 25. This configuration further includes two horizontal V-shaped brace members between the arcuate member and the C-shaped channel member. Manual adjustment of the blade angle is disclosed, wherein the blade pivots around axis 14 through the C-shaped channel member and is locked in a desired position. In Ciula U.S. Pat. No. 4,803,790, a C-shaped support member 26 is illustrated having no arcuate supporting structure. In applications employing light weight snowplow blades, as disclosed in Ciula, a single C-shaped support member has proven adequate. However, larger and/or more massive snowplow blades require more supporting strength, particularly for highway plowing applications where the vehicle speed exceeds that of typical parking lot or driveway operations. Adding further supporting braces and members, as shown in the Miceli and King patents, adds further cost, complexity, and weight to the system. Other methods of strengthening the C-shaped support member of Ciula include selection of different materials and increasing the thickness. These methods, however, further add to the cost and weight of the blade support apparatus, and are therefore undesirable. Optional support for vehicle snowplow blades is provided by skids as shown in U.S. Pat. Nos. 3,400,475; 3,432,946; and 3,432,947 to Peitl, or by casters as shown in U.S. Pat. No. 4,528,762 to Sarka, et al. In these systems, the skid or caster may be vertically adjusted by rotating a threaded shaft such as is shown in Peitl U.S. Pat. No. 3,400,475, and fixed by tightening a nut on the shaft. Alternatively, the vertical spacing between the skids or casters and the plow blade may be adjusted using a vertical skid shaft inserted within a vertical supporting plate, wherein horizontal holes are provided in the shaft and the plate for receiving a pin, as shown in Peitl U.S. Pat. No. 3,432,946. The use of a threaded shaft is an expensive method, as threaded surfaces must be provided both in the shaft as well as in the vertical structure to which the shaft is mounted. Horizontal holes and pins in combination provide relatively inexpensive vertical adjustment for skids or casters. However, such a pin must bear the vertical load of the snowplow mass, which can be very large for highway plows and the like. The pin must therefore be sized to accept the shear forces caused by the snowplow load. It is therefore desirable to provide a system for vertical adjustment of snowplow blade supporting skids, rollers, or casters which provides adequate vertical load handling capabilities with little or no increase in cost or complexity over the prior systems.